The present invention relates to a solenoid valve control having a switching output stage for controlling a solenoid valve, an arrangement for preselecting a valve opening time, and an arrangement for preselecting a pressure over the valve. The present invention also relates to a method of controlling a solenoid valve having the steps: controlling a solenoid valve having a switching output stage, preselecting a valve opening time and preselecting a pressure over the valve.
Various systems for automatically influencing the driving properties of a motor vehicle are known. These include, for example, the anti-skid system (ABS) and anti-spin control (ASC). In these systems, the pressure in a wheel brake cylinder is adjusted by solenoid valves whose switching states are influenced by a solenoid valve control. For example, in the brake system of a motor vehicle there are reversing valves (USV valves) on the path from a hydraulic pump to a master brake cylinder and inlet valves (EV valves) to a wheel brake cylinder for which solenoid valve controls are used. Both inlet valves and reversing valves, which can be operated linearly, are known. This property is utilized to an increasing extent in anti-skid systems (ABS) and anti-spin control (ASC) systems. In the case of linearly operated valves, the pressure drop over the valve is essentially a linear function of the electric valve current. Basically, the valve current can be adjusted through current-controlled valve output stages. However, such current-controlled valve output stages are expensive, so there has been a search for other options. It is already known that the valve current can be switched with inexpensive switching output stages. These switching output stages supply the electric valve voltage with pulse width modulation. A mean current can be adjusted by way of such pulse width modulation, using a freewheeling diode connected in parallel to the valve coil, so the desired pressure drop over the valve can be adjusted in this way. The freewheeling diode permits a current to flow when the output stage is turned off.
However, a disadvantage of the required freewheeling diode is that brief opening and subsequent closing of the valve are influenced to a great extent by the freewheeling diode. The reason for this great effect is that the current drops slowly when the control of the valve is shut down in comparison with a situation without a freewheeling diode. Since the switching threshold of a valve depends greatly on the pressure drop over the valve, the effect on the current characteristic due to the freewheeling diode described here leads to different switching times, depending on the pressure drop over the valve. This has resulted in different valve opening times, depending on the pressure drop over the diode, with identical valve control times.
Thus, on the whole, it is problematical to implement a pulse width-modulated control such as an LMV control (linearized solenoid valve) or CPC operation (continuous pressure control) in parallel with a pulse step control in a single solenoid valve control, because the freewheeling diode which is required for pulse width-modulated control prevents accurately implementable valve opening times. However, under some circumstances it may be desirable to use both pressure modulation principles in one controller, mainly against the background of preventing use of expensive current-regulated valve output stages. For example, it is possible to operate an anti-skid system (ABS) with a pulse width-modulated CPC on the inlet valve. In the case of anti-spin control (ASC), however, this is not possible because requirements regarding the accuracy of the set pressures are higher in anti-spin control (ASC).
The present invention is based on the generic solenoid valve control such that means are provided for determining a valve control time from the preselected valve opening time and the preselected pressure over the valve. In this way, it is possible to influence the variation in the valve opening time as a function of the pressure drop over the valve, namely by determining a xe2x80x9ccorrectedxe2x80x9d valve control time. The valve control time thus takes into account the valve current characteristic over time, for example, and therefore the effect of the switching time on the basis of the dependence of the switching thresholds on the pressure drop.
The means for preselecting a valve opening time preferably use an inverse hydraulic model. Hydraulic models are generally used in controllers to calculate a pressure on the basis of input parameters such as a valve current and a valve opening time, for example. With an inverse hydraulic model, it is possible to calculate a required valve opening time from the pressure over the valve.
The means for preselecting a pressure over the valve preferably use a pressure estimation model. Since the pressure over the valves cannot be determined directly, pressure estimation models which are contained in controllers of the related art are suitable means for estimating the required pressure for further determination of the valve control time.
It may be advantageous if the means for determining a valve control time use a table which lists pressure values and the respective correction values. Each pressure value can thus be assigned a certain correction value, for example, which transforms the preselected valve opening time into a valve control time. It is also conceivable for each pressure value to be assigned multiple correction values, one of the correction values being used to determine the time for opening the valve, for example, and another correction value being used for the time for closing the valve.
However, it may also be advantageous if the means for determining a valve control time model the time dependence of the valve current and the current dependence of the valve opening pressure. The required valve control times can thus be determined by determining functional relationships between the valve current and time or between valve opening pressure and current. The functional relationships can be determined mathematically or empirically in good approximation.
For example, the fact that the valve current may have an essentially exponential time dependence may be utilized. Mathematical models can be constructed easily with such exponential relationships.
It may also be advantageous if the means for determining the valve control time use an approximation function. The valve control time can be determined approximately by using approximation functions, this being sufficient for numerous applications.
A freewheeling diode is preferably connected in parallel with the solenoid valve, thus yielding LMV operation or CPC operation with pulse width-modulated voltage in parallel with pulse step control. In CPC operation, pulse width modulation takes place continuously, whereas in LMV operation, pulse width modulation takes place in phases, while at other times the valve is closed. Since a freewheeling diode in parallel with the solenoid valve is used for both modes of operation, the pressure dependence of the switching times of the valve is especially great. Therefore, the present invention manifests its advantages especially in such parallel operation of pulse step control and pulse width-modulated control.
The present invention also manifests its particular advantages due to the fact that there is CPC operation at the inlet valve of a brake cylinder for an anti-skid system control (ABS control), and there is pulse step control at the inlet valve of a brake cylinder for an anti-spin control (ASC) system. It is thus possible to eliminate current-regulated valve output stages in an inexpensive manner. For ABS control, pulse width-modulated CPC operation may be used at the inlet valve of the brake cylinder. This is not possible with anti-spin control (ASC), if no current-regulated valve output stages are to be used at the inlet valves.
The present invention is based on the generic method such that a valve control time is determined from the preselected valve opening time and the preselected pressure over the valve. In this way, it is possible to influence the variation in the valve opening time as a function of the pressure drop over the valve, namely by determining a xe2x80x9ccorrectedxe2x80x9d valve control time. The valve control time thus takes into account the valve current characteristic over time, for example, and therefore the effect of the switching time on the basis of the dependence of the switching thresholds on the pressure drop.
A valve opening time is preferably preselected by using an inverse hydraulic model. With an inverse hydraulic model, it is possible to calculate a required valve opening time from the pressure over the valve.
A pressure over the valve is preferably preselected by using a pressure estimation model. Since pressure over the valves cannot be determined directly, pressure estimation models which are contained in controllers of the related art are suitable means for estimating the required pressure for further determination of the valve control time.
It may be beneficial if a valve control time is determined by using a table which lists pressure values and the respective correction values. Each pressure value can thus be assigned a certain correction value, for example, which transforms the preselected valve opening time into a valve control.
However it may also be beneficial if the time dependence of the valve current and the current dependence of the valve opening pressure are modeled when determining a valve control time. The required valve control times can thus be determined by determining the functional relationships between the valve current and the time or between the valve opening pressure and the current. The functional relationships can be determined mathematically or empirically in good approximation.
It is especially beneficial for the modeling if the valve current has an essentially exponential time dependence. Mathematical models can be constructed easily with such exponential relationships.
However, it may also be beneficial if a valve control time is determined by using an approximation function. The valve control time can be determined approximately by using approximation functions, this being sufficient for numerous applications.
The present invention is especially advantageous due to the fact that a freewheeling diode is connected in parallel with the solenoid valve, thus yielding LMV operation or CPC operation with pulse width-modulated voltage in parallel with pulse step control. In CPC operation, pulse width modulation takes place continuously, whereas in LMV operation, pulse width modulation takes place in phases, while at other times the valve is closed. Since a freewheeling diode in parallel with the solenoid valve is used for both modes of operation, the pressure dependence of the switching times of the valve is especially great. Therefore, the present invention manifests its advantages especially in such parallel operation of pulse step control and pulse width-modulated control.
However, the present invention also manifests particular advantages due to the fact that there is CPC operation at the inlet valve of a brake cylinder for an anti-skid system control (ABS control) and there is pulse step control at the inlet valve of a brake cylinder for an anti-spin control (ASC) system. It is thus possible to eliminate current-regulated valve output stages in an inexpensive manner. For ABS control, pulse width-modulated CPC operation may be used at the inlet valve of the brake cylinder. This is not possible with anti-spin control (ASC), if no current-regulated valve output stages are to be used at the inlet valves.
The present invention is based on the surprising finding that the valve control time can be determined in the case of a switching output stage even with valve opening thresholds that have a great dependence on pressure. This has advantages with regard to the combination of pulse width-modulated control with pulse step control, because in pulse width-modulated control, a freewheeling diode is connected in parallel with the valve coil, which leads a great influence over time on the current characteristic.